A lot of adventure fiction involves far more to it than easily modeled practical puzzles and combat; getting from point A to point B and traversing the obstacles in between makes up a good chunk of it. Unfortunately, it’s one of the hardest to model in game.

Fafhrd & the Gray Mouser don’t have near the trouble fighting bandits, thieves, and evil priests, pilfering ruined castles and temples, or escaping from the clutches of evil wizards as they do spending several days climbing up an almost sheer granite cliff-face, carving out foot and hand-holds with an ax, using grapnels, pikes, and pitons to climb around lips and overhanging boulders, and trying not to starve or die of exposure. Even without the ethereal fire princesses haunting them, mountain goats stampeding at them, invisible manta-ray riders hounding them, furry dragons harrying them, rival adventurers taking the occasional pot-shot at them, the duo have their work cut out for them just getting to the top.

The problem is, if you’re wanting to model an adventure after any of those “man against the elements” type stories, you’ll have your work cut out for you too. Overland travel is often one of the most hand-waved elements of Dungeons & Dragons; it’s hard to make it gameable. Rules exist for getting lost, determining weather and possibly even contracting disease, but getting lost screws up mapping (something DMs are loathe to do to their players, as mapping is challenging enough as it is sometimes), weather is window-dressing, and saving throws to not get sick aren’t much fun (and they’re a passive mechanic).

Outdoor Survival offers some excellent mechanics to mini-game overland travel, and it’s obvious why it would be used as core component of the original D&D system, but it offers little for the sort of adventure that involves a lot of climbing.

Thieves in D&D have some pretty impressive cooked in mechanics for scaling sheer surfaces that may be as much from Fafhrd & Mouser as they are from Jack of Shadows; if a thief can only use their thief skills with thief tools, could those thief tools include climbing instruments? Fafhrd & Mouser have some impressive gadgets they use which, while they don’t make the climb ‘easier’, make the climb at least possible.

So, you want to have an adventure with treasure on top of a mountain that players will have to climb, you want the challenge of climbing the mountain to be a significant part of the adventure, and you don’t want the climb to be just a series of rolls and skill checks.

This will probably take a lot of planning, because to make the climb gameable, you’ll have create unique challenges for the climb, and some of them probably WILL inevitably involve skill or stat checks.

You’ll want to do some research on equipment and specify what the players will have available to them, and possibly give them instruction on how to use it; if they know what it’s for, they’ll have a better shot at solving puzzles with them.

For instances where you DO need to rely on skill checks, have the result lead to a new situation that can be dealt with via problem solving; a gust of wind or a slipped hold might not result in the whole party falling off the mountain right away, but someone might come loose and need to be pulled up by others, or if someone ends up suspended free, they’ll have to figure out how to swing back to the cliff face and get hold.

Because it’s D&D, you’re probably going to want to break up the climbing with encounters; or even throw in some encounters during the climb. Having to fight off some bird or while clinging to a rock-face could create some unique challenges for players. But it’s likely to be lethal. This is the sort of adventure that has TPK written all over it; wrong moves mean death, characters can only carry with them the absolute essentials, and there’s no reward for making it part-way, so be sure that whatever you put on top of that mountain is damn well worth it.

Suggestions:

Do some equipment research; come up with some reasonable tools for the players to use plus some stuff that they can come up with their own tricks with.

Create several specific challenges and mechanics to adjudicate them, including set-pieces challenges, encounters, and general progress challenges.

Create general and specific weather effects: what will gusts of wind do to the climbers? How will snow affect climbing? How will you handle things like fatigue and exposure?

Plot out your mountain; the ascent will be a chain of the challenges you create. To make things more interesting, offer multiple paths that branch between tiers, allowing players more agency over their ascent. Perhaps one ascent will appear more straight-forward but passes by the lair of a monster, while another ascent avoids it entirely but requires climbing up an overhanging lip.

Be sure to give your players a few things to do and look at on the way up; a monster lair to explore or just a spot to set-up camp and recover (perfect for a spooky night encounter). And remember how I said there was no reward for making it part-way? If the players end up not wanting to throw their characters away, there might be some value at least in hunting beasts living in crevasses in the mountain; it is D&D, after all. Still, the real goal should be the summit.